The present invention relates to semiconductor lasers, in particular to Quantum Cascade Lasers (QCLs) emitting in the IR spectral range, i.e. at wavelengths of 1 mm to 780 nm, especially in the mid IR range of 3-50 μm.
The mid-IR spectral range is important for sensing applications due to the large number of molecules showing fundamental resonances in this region. Quantum Cascade Lasers (QCLs) have become frequently used and efficient laser sources for such applications. Examples are Maulini U.S. Pat. No. 7,944,959 and Faist US patent application 2003/0 174 751. A QCL laser generating a mid-IR spectrum is also shown in Vurgaftman U.S. Pat. No. 8,290,011.
Typical lateral-guiding structures used to form waveguides for quantum cascade lasers (QCLs) are deep-etched ridge waveguides, shallow-etched ridge waveguides or deep buried heterostructure (BH) waveguides. For high power and high performance devices, the buried heterostructure configuration is favored since it presents a higher thermal conductivity of the InP burying layers and at the same time guarantees low losses. Beck U.S. Pat. No. 6,665,325 is an example.
More specifically, the current blocking in the case of the Fe-doped InP is guaranteed by the built-in potential present at the interface between the Fe-doped InP and the n-doped InP contact layer. Since Fe acts as a deep donor level in InP, it also actively helps to trap electrons and thus prevents leakage currents. The magnitude of the built-in potential can be controlled by adjusting the Fe-doping in the InP. Unfortunately the latter parameter is strongly connected to the growth parameters and the built-in potential is typically limited to 50-100 kV/cm. Typically in the case of QCLs operating in a high electric field, the current is therefore partially driven inside the insulating burying layers. This effect reduces the actual current injected into the laser active region and, at the same time, it dissipates heat and therefore degrades laser performance.
In addition, it is well known that the introduction of Fe impurities inside the burying layers creates defect states inside the semiconductor forbidden gap, see P. B. Klein et al., Phys. Rev. B 29, 1947 (1984): Time-dependent photo-luminescence of InP:Fe. This levels present absorptions, especially in the 3-4 μm spectral range, preventing the production of buried-heterostructure laser devices in this range.
This spectral region however is of crucial interest for spectroscopy and sensing applications due to the presence of fundamental resonances of many molecules in this spectral region. One of the aims of the present invention is to overcome the above-mentioned limitation and devise a method for making BH lasers with low optical losses also in this spectral range.
However, it should be understood that the present invention is not limited to QCLs of this wavelength, but is generally applicable for QCLs across the spectral range, e.g. to any BH laser design, be it a QCL with multi-color emitters or any other BH laser.